Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
directly involved in ice formation is difficult to obtain from in situ measurements. On the
other hand, fresh soot particles do not act as efficient ice (deposition) nuclei in the exhaust
(Rogers et al., 1998), consistent with the absence of contrails at temperatures above the
liquid water saturation threshold.
Contrails observed near threshold formation conditions are thought to result from freezing of
water on soot particles (Kärcher et al., 1996b; Schumann et al., 1996; Brown et al., 1997)
(Figure 3-5). This result is supported by laboratory experiments (DeMott, 1990; Diehl and
Mitra, 1998) that provide evidence that soot may induce ice formation by heterogeneous
immersion freezing at temperatures colder than about 250 K. Water activation of soot may
result from the formation of at least a partial surface coating of H 2 SO 4 /H 2 O droplets, which
likely develops for average to high fuel sulfur levels (Figure 3-5). Hence, more fuel sulfur
leads to a greater number of ice particles. However, observations demonstrate that the
number of ice particles (diameter > 300 nm) in young contrails increases by only about 30%
when the fuel content increases from 6 to 2700 ppm (Petzold et al., 1997), as model
simulations of contrail formation also show (Kärcher et al., 1998a).
Contrails at threshold conditions appear to be formed for very low (2 ppm) fuel sulfur content
in the same manner as for average fuel sulfur content (260 ppm) (Busen and Schumann,
1995), but their properties differ measurably for larger fuel sulfur content (Schumann et al.,
1996). This result suggests that soot may take up water even at zero fuel sulfur content,
though this uptake may be enhanced in the presence of sulfur emissions (Kärcher et al.,
1998a; see Figure 3-5).
Figure 3-5: Soot activation and heterogeneous freezing in young
The presence of liquid coatings may alter the chemical reactivity of dry exhaust soot, which aircraft exhaust plumes (Kärcher, 1998a).
is poorly known (Chapter 2). Soot particles acting as freezing nuclei have the potential to
alter cirrus cloud properties (see Section 3.4). Present observations do not rule out the
possibility that aircraft soot particles can act as freezing nuclei in cirrus formation, perhaps even without a H2SO4 /H2O coating. Information is lacking on how the
chemical reactivity and freezing properties of soot might change in aging plumes from interactions with background gases and particles or as a result of aerosol
processing in contrails.
Table 3-2: Emission indices and estimated global emission rates of exhaust products of the present (1992) aircraft fleet using representative emission
indices. Emission sources other than aircraft and estimated magnitudes of these emissions are listed in the last two columns. Values in parentheses indicate
estimated range (adapted from Fabian and Kärcher, 1997; Schumann, 1994).
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